1. Field of the Invention
The present invention relates to a method and means for improving the efficiency of light generation by an electrodeless fluorescent lamp and, more particularly, to a method and means for increasing visible light production by an electrodeless fluorescent lamp of the order of 50% to 100%.
2. Description of the Prior Art
The incandescent lamp is a principal source of lighting in homes and businesses. However, its light emitting filament evaporates and becomes weak with use, hence it is easily fractured or dislodged from its supports. Thus, the lifetime of an incandescent lamp is short and unpredictable. More importantly, the efficiency of an incandescent lamp in converting electrical power to light is very low, e.g. approximately 15 lumens of light per watt of electrical power.
Fluorescent lamps are generally more efficient and durable than incandescent lamps. However, a conventional fluorescent lamp requires both a ballast supply and a special fixture which must be installed before a lamp can be used. Furthermore, the electrodes are subject to disintegration over a period of time.
In my copending U.S. Patent Application Ser. No. 604,226, filed Aug. 8, 1975, and entitled Light Generation By an Electrodeless Fluorescent Lamp, there is disclosed a method and means for producing light from an electrodeless discharge established in an ionizable medium within a sealed envelope. The sealed envelope has therein at least one particular ionizable gas at a given pressure capable of emitting radiant energy when subjected to a radio frequency field. After initiating an electrodeless discharge in the medium, the discharge is maintained by coupling to the medium a radio frequency magnetic induction field having a frequency and magnitude selected to optimize the efficiency of conversion of radio frequency power to light.
Such a lamp eliminates the disadvantages of both incandescent and conventional fluorescent lamps. On the one hand, it eliminates filaments, ballast supplies, and special fixtures. On the other hand, it is capable of efficiencies similar to those conventional fluorescent lamps.
According to the preferred embodiment of that invention, a sealed, light transmissive, envelope including an outer wall which can be shaped as an incandescent lamp bulb is charged with mercury vapor and an inert starting gas, such as argon. Positioned within the outer wall is an elongate inner wall defining an open-ended cavity. A layer of fluorescent light emitting phosphor is disposed on the interior surface of the outer wall. An induction coil is positioned within the open-ended cavity so that a substantial portion of its magnetic induction field passes through the medium. A radio frequency oscillator having a tuned circuit including the coil in series with a capacitor is housed within a base that screws into a conventional incandescent lamp socket. The oscillator generates electrical energy at a radio frequency which is applied to the coil to produce both an electrical field and a magnetic field. The electric field across the coil initiates ionization of the mercury vapor in the envelope and the magnetic induction field maintains such ionization so as to emit ultraviolet light that excites the phosphor to emit visible light.
During the normal operation of such a lamp, it has been found that part of the ultraviolet energy emitted from the electrodeless arc impinges upon the inner wall where it is absorbed by the glass and lost to the system. Provision for a specular reflector on the outer surface of the inner wall, such as may be formed by vapor deposition of aluminum or the like, tends to effectively short-circuit the discharge starting field and, in the absence of at least one longitudinal break in the electrically conductive reflective surface, may also short out the induced discharge maintenance field. On the other hand, if such a coating is provided on the inner surface of the inner wall, the same problem exists together with the additional problem of ultraviolet absorption by the glass inner wall.
Likewise, an external reflector derived from the concept presented in my prior U.S. Pat. No. 3,860,854 provides no viable solution for identical reasons. In any event, all of these means are costly and therefore undesirable if alternate, less expensive, effective means are available.